1. Field of the Invention
The present invention relates to a submicron level dimension reference for controlling the precision of a scanning electron microscope (SEM) which is employed for the purpose of controlling the critical dimensions (CD) in manufacturing a semiconductor device. More specifically, the submicron level dimension reference prevents an electrification or charging problem while using the SEM in manufacturing the semiconductor device.
2. Discussion of Related Art
When manufacturing semiconductor devices, a scanning electron microscope (SEM) is generally used for measuring critical dimensions (CD) of the semiconductor device. Generally, a SEM measures the sizes of microscopic objects. Accordingly, the precision of the SEM is very important and must be adjusted periodically to be kept at a proper level.
When a reference whose size and pattern are already known is measured with the SEM, the measured value is compared with the known value. If the two values are different, the SEM should be adjusted to make them identical through the fine manipulation of the SEM. The known reference, used as a basis for adjusting the SEM, is a submicron level dimension reference, of which there are three conventional types.
The first type of a submicron level dimension reference 10 is a poly pattern type shown in FIG. 1. In this type, silicon dioxide 12 is deposited on a silicon substrate 11 so that the adhesion between a polysilicon 13, which is deposited on the silicon dioxide layer 12, and the silicon substrate 11 is increased. The polysilicon 13 is deposited over the silicon dioxide 12 and forms a pattern.
The second type of submicron level dimension reference 20 is a silicon dioxide pattern type shown in FIG. 2. In this type, silicon dioxide 22 is deposited on a silicon substrate 21 so that the adhesion between the silicon substrate 21 and a conductive layer 23 (e.g., polysilicon) deposited on the silicon dioxide 22 is increased. Another silicon dioxide layer 24 is formed on the conductive layer 23 and forms a pattern.
The third type of submicron level dimension reference 30 is a standard micro scale type shown in FIG. 3. In this type, a pattern is formed on the silicon substrate 31 itself. Its width and length are 1 cm, respectively. This reference type is manufactured by HITACHI of Japan and approved by the Japan Quality Assurance Organization.
These submicron level dimension references must meet several requirements before being used for adjusting the SEM.
First, the pitch or the size of reference pattern must be confirmed by a precise measuring apparatus, other than the specific SEM which will be adjusted by the particular reference.
Second, when the reference is measured in an electron beam metrology system, such as that of a SEM, a secondary electron signal must be generated with high contrast. In other words, the size or reference pattern must be detected precisely and the pattern itself must be formed precisely.
Finally, the reference must be stable and be free of charging (electrification) as it will be used for long periods of time in the presence of an electron beam.
The conventional submicron level dimension references have some problems in meeting these criteria.
In the cases of the poly pattern 10 (FIG. 1) and silicon dioxide pattern 20 types (FIG. 2), the reference becomes overcharged due to the electrons accumulated during the operation of the SEM. These accumulated electrons on the reference change the paths of the electrons projected from the SEM, which changes the size and pattern values of an object that is measured by the equipment (SEM), making it difficult to control the equipment precisely. Specifically, the nonconductor silicon dioxide between the silicon substrate and the conductor polysilicon does not allow the charge accumulated on the surface of the polysilicon to flow toward the substrate, so that the measured value of the CD using the reference is different from the known value.
This shortcoming is illustrated in FIG. 4, which is a SEM display of a submicron level dimension reference of the silicon dioxide pattern type 20 as in FIG. 2. This display shows a view of the reference 20 and the measured SEM pattern 29. The SEM measured pattern 29 resulted from scanning the surface area designated "1" at high power for 60 seconds, followed by scanning surface areas "1" and "2" at low power.
In this scanning process, electrons projected from the SEM are accumulated and charged on the previously measured narrow area "1". Although the actual area of "1" and "2" together is flat and they are composed of the same material, the previously measured narrow area "1" appears much darker than area "2" as shown in FIG. 4. Moreover, in the SEM pattern 29, this narrow area "1" also appears lower than the surrounding area "2". This display clearly shows the distortion of the reference's pattern caused by the charging effect.
There is no charging problem in the standard micro scale type 30 of FIG. 3, but the pattern is too simple to accurately perceive the variation of the SEM as it is measuring the CD.
For example, when using references 10 and 20, if the setting of the SEM is changed slightly, you are made aware that the SEM must be adjusted because the depth of focus is obscured in the SEM. However, if you use the standard micro scale type 30, you are not made aware of the necessity to adjust the SEM because the simplicity of the pattern does not provide sufficient pattern contrast in the SEM display.
As a result of these shortcomings, SEM measurements are not performed in an optimal manner, leading to imprecise measured values that are different than the known values.
Another disadvantage of these conventional references is that it is difficult to install them directly in the SEM because the shape of the reference is not the same as the wafer used in the semiconductor device manufacturing process. The reference may also be contaminated easily while an operator manually adjusts the equipment to suitably receive the reference. Another disadvantage is that the reference may not be used for long periods because of contaminants from the surroundings.